1. Field of the Invention
The present invention relates to a thermal fuse selecting and using, as a thermosensitive material, a polyolefin wax typified by polyethylene and polypropylene, each of which are a thermoplastic wax.
2. Description of the Background Art
The thermal fuse employing a thermosensitive pellet using a non-conductive thermosensitive material (hereinafter simply referred to as a thermosensitive material) is a so-called irreversible type thermal switch which generally operates at a prescribed temperature at which the thermosensitive material formed into a pellet shape is softened and melted, to interrupt the current conducting path of equipment and an apparatus for protection thereof. It operates at a temperature (operating temperature) approximately determined by the composition of the thermosensitive material to be used. And a switching component such as a spring is selected to adjust the operating temperature. For example, the thermal fuse employing a thermosensitive pellet is configured such that the non-conductive thermosensitive material is pelletized and used in a metal casing having a lead attached at each end, in which a switching member including a spring member such as a compression spring and a slidable contacting conductor is housed in the predetermined position of the metal casing. The thermosensitive material to be pelletized is selected from a pure chemical substance alone, a thermoplastic resin material used alone or a thermoplastic resin composition. In this case, the thermosensitive pellet softens or melts at a prescribed operating temperature and the compression spring moves the contact of the movable conductor to thereby cause insulation between a pair of leads. The thermosensitive material of the thermosensitive pellet is pelletized by the prescribed forming process including granulation and tableting. A thermoplastic resin material is recently used as the thermosensitive material, which is characteristically advantageous because of an improvement concerning the adjustment of the operating temperature.
Japanese Patent Laying-Open Nos. 2003-317589, 2005-158681 and 2006-260926 (Patent Documents 1 to 3) disclose improved structures of a thermal fuse employing a thermosensitive pellet using a non-conductive thermoplastic resin material for a thermosensitive material in accordance with the proposal by the inventor of the present invention. These provide a method for solving some problems concerning the workability and operating characteristics in the case where the thermoplastic resin is used as a thermosensitive material. For example, these present selection of the resin material, means for adjusting the operating temperature, and appropriate means against the characteristic degradation associated with the change over time.